1. Field of the Invention
The invention relates to the manufacture of highly dense integrated circuits, and more particularly to the formation of a self-aligned contact between interconnecting conductive lines and devices within the integrated circuit.
2. Description of the Related Art
In the manufacture of highly dense integrated circuits, semiconductor devices formed in and on a silicon substrate need to be connected to each other, and to other devices or connections external to the chip on which the device is formed. Interconnections are formed of a conductive film, such as polysilicon or aluminum. Desirable characteristics of these films include, among others, low resistivity, stability throughout processing, good adherence, and ease of formation and patterning. As device dimensions have steadily been reduced, shallower junctions are formed in the active regions, which places further demands on the contact and interconnect metallization. Avoidance of spiking shorts, good step coverage for narrow contact windows and low contact resistance become important characteristics for the shallow junctions and small lateral dimensions of devices formed at dimensions in the range of about 1.0 micrometer and less.
For contacts, aluminum has been satisfactory at larger dimensions due to its ease of processing, ability to reduce native silicon dioxide, and low resistivity. However, at smaller dimensions, aluminum leads to spiking shorts (due to deep penetration of metal shorts into the semiconductor) and contact depths that are a significant fraction of the junction depth. Tungsten is a possible substitute for aluminum interconnections. However, its resistivity of 8-10 microohms-centimeters (um-cm) is about 3 times that of aluminum (2.7 um-cm), so the tungsten thickness needs to be minimized.
For these and other reasons, it has been suggested to use tungsten plugs in the contact windows, and connecting to the plugs by a subsequent deposition of aluminum to form conductive lines. For example, in U.S. Pat. No. 4,713,356 (Hiruta), U.S. Pat. No. 4,804,560 (Shioya et al), and U.S. Pat. No. 4,898,841 (Ho) tungsten is used to fill the contact holes and make contact with the device active regions, and aluminum formed to connect to the tungsten plugs. However, this requires accurate alignment to insure connection between the aluminum and tungsten, and the metal overlap contact is limited by alignment accuracy.